The tip size wont really make a difference in anything but the sound...A larger tip will make it deeper, and smaller will increase the pitch...
...You could go into the fact that a larger tip will cause the exhaust gas to become turbulent as it exits the smaller pipe, and therefore the velocity should decrease, but since the tip is so short, it becomes almost a moot point...It is this slowing of the gases in a large tip that causes the deepening of the exhaust note, however...
...And, the transition from 2.25 into 2" pipe isnt very smooth...Its actually a 'step' right after the second cat...It induces some turbulence into the exhaust flow...If it was a smoother transition, my ramblings about velocity and flow and such would be much more valid...I was merely proposing theory...
Go nuts with the tip size...For all intents and purposes, it wont make a difference in velocity...